Method and apparatus for increased communication channel pre-emphasis for clock-like data patterns

ABSTRACT

Methods and apparatus are disclosed for increased pre-emphasis for clock-like data patterns to compensate for channel distortions. One aspect of the invention compensates for channel distortions by evaluating a data pattern to be transmitted; determining if the data pattern satisfies one or more predefined criteria defining a clock-like data pattern; and generating a pre-emphasis level for the clock-like data patterns that is higher than a pre-emphasis level for the data patterns that do not satisfy the one or more predefined criteria. For example, a predefined window size can be defined for determining if the data pattern satisfies the one or more predefined criteria defining the clock-like data pattern. In one exemplary implementation, the higher pre-emphasis level is generated for one or more predefined data patterns. A table can optionally be accessed to determine the pre-emphasis level based on the data pattern.

FIELD OF THE INVENTION

The present invention is related to techniques for compensating forchannel distortions and, more particularly, to improved pre-emphasistechniques that compensate for channel distortions.

BACKGROUND OF THE INVENTION

Digital communication receivers often sample an analog waveform and thenreliably detect the sampled data. Signals arriving at a receiver aretypically corrupted by intersymbol interference (ISI), crosstalk, echo,and other noise. In order to compensate for such channel distortions,communication systems often employ well-known pre-emphasis techniques inthe transmitter or equalization techniques in the receiver (or both). Onthe receiver side, well-known zero equalization or decision-feedbackequalization (DFE) techniques (or both) are often employed.

A communication channel typically exhibits a low pass effect on atransmitted signal. Conventional pre-emphasis employed by a transmitterattempt to open the received data eye that has been band limited by thelow pass channel response. Thus, the channel will generally impair thehigher frequency components of a transmitted signal more than the lowerfrequency components. While existing pre-emphasis techniques effectivelycompensate for channel distortions, they suffer from a number oflimitations, which if overcome, could further improve the reliability ofdata detection in the presence of channel distortions.

A need exists for improved pre-emphasis techniques that amplify the highfrequency content of transmitted data. A further need exists for methodsand apparatus for applying increased pre-emphasis to higher frequencycomponents of a transmitted signal, such as clock-like data patterns.

SUMMARY OF THE INVENTION

Generally, methods and apparatus are disclosed for increasedpre-emphasis for clock-like data patterns to compensate for channeldistortions. One aspect of the invention compensates for channeldistortions by evaluating a data pattern to be transmitted; determiningif the data pattern satisfies one or more predefined criteria defining aclock-like data pattern; and generating a pre-emphasis level for theclock-like data patterns that is higher than a pre-emphasis level forthe data patterns that do not satisfy the one or more predefinedcriteria. For example, a predefined window size can be defined fordetermining if the data pattern satisfies the one or more predefinedcriteria defining the clock-like data pattern. In one exemplaryimplementation, the higher pre-emphasis level is generated for one ormore predefined data patterns. A table can optionally be accessed todetermine the pre-emphasis level based on the data pattern.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically illustrates a number of ideal data eyes associatedwith a signal;

FIGS. 2A and 2B illustrate the distortion that can arise from a channel;

FIG. 3 illustrates an exemplary signal flow for a channel compensationtechnique implemented in accordance with the present invention;

FIG. 4 illustrates the application of pre-emphasis in accordance withthe present invention for an exemplary 3-tap finite impulse response(FIR) filter;

FIG. 5 is a sample table describing an exemplary pre-emphasis assignmenttable 500 incorporating features of the present invention;

FIG. 6 is a schematic diagram of an exemplary pre-emphasis circuithaving three current sources I₁, I₂ and I₃ that may be employed in oneembodiment of the invention; and

FIG. 7 is a schematic diagram of an exemplary pre-emphasis circuithaving four current sources I₁, I₂, I₃ and I₄, that may be employed inone embodiment of the invention.

DETAILED DESCRIPTION

The disclosed pre-emphasis techniques amplify the high frequency contentof transmitter data. In addition, the disclosed methods and apparatusapply increased pre-emphasis to clock-like data patterns. In oneexemplary embodiment, the data pattern is observed and a pre-emphasislevel is selected for the current bit based on the observed datapattern.

FIG. 1 graphically illustrates a number of ideal data eyes 110-1 through110-3 associated with a signal 100. Although the ideal data eyes 110shown in FIG. 1 do not exhibit any noise for ease of illustration, eachdata eye 110 is typically a superposition of a number of individualsignals with varying frequency components, in a known manner. The timebetween the approximate center of two zero-crossing points 120-1, 120-2corresponds to the unit interval of the data eye. It is noted that inthe presence of a noisy signal, such as noise resulting from significantchannel distortion, the data eyes 110 will exhibit a wider zero crossingpoint. There is an inverse correlation between the statistical variationbetween the zero-crossing points 120-1, 120-2 and the degree of opennessof each data eye 110. Thus, as the width of the zero-crossing points120-1, 120-2 increases, the degree of openness of each data eye 110decreases.

FIGS. 2A and 2B illustrate the distortion that can arise from a channel.FIG. 2A illustrates an ideal transmitter output 200. FIG. 2B illustratesthe data eye 210 at the receiver that results from the channeldistortion, when no channel compensation is applied. As evident in FIG.2B, the data eye 210 is essentially closed due to the channeldistortion.

The present invention recognizes that existing pre-emphasis techniquescan be improved by applying increased pre-emphasis to higher frequencycomponents of a transmitted signal, such as clock-like data patterns.While existing pre-emphasis techniques apply the same pre-emphasis forall data patterns, the present invention enhances the high frequencycomponents of clock like data patterns by increasing the pre-emphasisfor clock like data patterns. In one exemplary implementation, discussedfurther below in conjunction with FIG. 7, the increased pre-emphasis isobtained by using an additional current source.

FIG. 3 illustrates an exemplary signal flow 300 for a channelcompensation technique implemented in accordance with the presentinvention. As shown in FIG. 3, pre-emphasis techniques 310 are appliedin the transmitter before the signal is transmitted over a channel 320.In addition, equalization techniques 330, such as zero equalization, areoptionally applied in the receiver. An optional cross over monitor 340implements a channel compensation parameter optimization process, forexample, as described in U.S. patent application Ser. No. 11/434,687,filed May 16, 2006, entitled “Method and Apparatus for Determining Oneor More Channel Compensation Parameters Based on Data Eye Monitoring,”to determine when one or more of the pre-emphasis 310 and equalization330 have sufficiently compensated for the channel distortion. Whenpre-emphasis techniques 310 are applied in the transmitter, the outputof the cross over monitor 340 is fed back to the transmitter using anin-band or out of band protocol 350.

FIG. 4 illustrates the application of pre-emphasis (clock patternemphasis) in accordance with the present invention for an exemplary3-tap finite impulse response (FIR) filter. After a transmitted signaltravels through a channel having a low pass effect, the high frequencyboost associated with the pre-emphasis techniques of the presentinvention will compensate for channel impairments and the desiredfrequency content of the data will be retained. The performance of thepre-emphasis process is improved by selectively giving additional boostfor clock like data patterns to further improve the existingpre-emphasis performance.

Generally, as shown in FIG. 4, when determining how much pre-emphasis toapply for a current bit, an exemplary implementation of the presentinvention evaluates the values of the preceding bit and the subsequentbit. Conventional pre-emphasis techniques employ three current sourcesin order to weight the lead, lag, and steady data bits. In exemplaryembodiments of the present invention, four pre-emphasis levels (steadydata, lead, lag and clock levels) are obtained using three or fourcurrent sources, as discussed further below in conjunction with FIGS. 6and 7, respectively. The signal level associated with each data patternis shown in FIG. 5.

FIG. 5 is a sample table describing an exemplary pre-emphasis assignmenttable 500 incorporating features of the present invention. As shown inFIG. 5, for each possible three bit data pattern of preceding, currentand subsequent bits, the table 500 identifies the pre-emphasis levelthat should be applied for the data bit. In addition, a correspondingcurrent source configuration is also provided that indicates thepolarity of each current source that is used to obtain the desiredpre-emphasis level (lead, lag, steady or clock).

FIG. 6 is a schematic diagram of an exemplary pre-emphasis circuit 600having three current sources I₁, I₂ and I₃ that may be employed in oneembodiment of the invention. The pre-emphasis circuit 600 is generatingthe data bit in a known manner. The value of control signals K₁ throughK₃ determine the position of corresponding switches and thus whether thegenerated current goes through a first or second resistor 610, 620. Inan algebraic current combination, each current source can be assigned apercentage of current, such as 80%, 20% and 10%, for an exemplarymaximum current of 110%. The pre-emphasis circuit 600 of FIG. 6 cangenerate three independent levels and a fourth level that depends on theother three levels.

The present invention first detects the data pattern, and then selectsan appropriate pre-emphasis level for the data bit based on theexemplary assignment of FIG. 5. For example, based on the exemplaryassignments in FIG. 5, to generate the “lead” pre-emphasis level,associated with a data pattern of 001, the first and second currentsources, I₁ and I₂, are placed in a negative position (through resistor620) and the third current source, I₃, is placed in a positive position(through resistor 610) to obtain the desired level (−I₁−I₂+I₃).

FIG. 7 is a schematic diagram of an exemplary pre-emphasis circuit 700having four current sources I₁, I₂, I₃ and I₄, that may be employed inone embodiment of the invention. The control signals, K, and currentsources, I, operate in the same manner as described above in conjunctionwith FIG. 6. The pre-emphasis circuit 700 of FIG. 7 can generate fourindependent levels, in a known manner.

A plurality of identical die are typically formed in a repeated patternon a surface of the wafer. Each die includes a device described herein,and may include other structures or circuits. The individual die are cutor diced from the wafer, then packaged as an integrated circuit. Oneskilled in the art would know how to dice wafers and package die toproduce integrated circuits. Integrated circuits so manufactured areconsidered part of this invention.

It is to be understood that the embodiments and variations shown anddescribed herein are merely illustrative of the principles of thisinvention and that various modifications may be implemented by thoseskilled in the art without departing from the scope and spirit of theinvention.

1. A method for compensating for channel distortions, comprising:evaluating a data pattern to be transmitted; determining if said datapattern satisfies one or more predefined criteria defining a clock-likedata pattern; and generating a pre-emphasis level for said clock-likedata patterns using a pre-emphasis circuit, wherein said pre-emphasislevel is higher than a pre-emphasis level for said data patterns that donot satisfy said one or more predefined criteria.
 2. The method of claim1, wherein said determining step further comprises the step ofdetermining if said data pattern satisfies said one or more predefinedcriteria defining said clock-like data pattern within a predefinedwindow size.
 3. The method of claim 1, wherein said generating stepfurther comprises the step of generating said higher pre-emphasis levelfor one or more predefined data patterns.
 4. The method of claim 1,wherein said evaluating and determining steps comprise the step ofaccessing a table to determine said pre-emphasis level based on saiddata pattern.
 5. The method of claim 1, wherein said determining stepfurther comprises the step of evaluating values of one or more adjacentbits within a predefined window.
 6. The method of claim 1, wherein saidone or more predefined criteria defining said clock-like data patterncharacterize whether said data pattern exceeds a predefined frequencyvalue.
 7. The method of claim 1, wherein said one or more predefinedcriteria comprises one or more predefined data patterns.
 8. The methodof claim 1, wherein said one or more predefined criteria characterizewhether said data pattern is a clock pattern or said clock-like datapattern within an observation window.
 9. The method of claim 1, whereina total number of pre-emphasis levels is comprised of said higherpre-emphasis level for said clock-like data patterns and saidpre-emphasis levels for said data patterns that do not satisfy said oneor more predefined criteria and wherein said total number ofpre-emphasis levels exceeds a total number of bits in an observationwindow.
 10. A circuit for compensating for channel distortions,comprising: a data pattern detector configured to evaluate a datapattern to be transmitted; and a pre-emphasis circuit configured to:determine if said data pattern satisfies one or more predefined criteriadefining a clock-like data pattern; and generate a pre-emphasis levelfor said clock-like data patterns that is higher than a pre-emphasislevel for said data patterns that do not satisfy said one or morepredefined criteria.
 11. The circuit of claim 10, wherein saidpre-emphasis circuit is further configured to determine if said datapattern satisfies said one or more predefined criteria defining saidclock-like data pattern within a predefined window size.
 12. The circuitof claim 10, wherein said pre-emphasis circuit is further configured togenerate said higher pre-emphasis level for one or more predefined datapatterns.
 13. The circuit of claim 10, wherein said pre-emphasis circuitis further configured to access a table to determine said pre-emphasislevel based on said data pattern.
 14. The circuit of claim 10, whereinsaid pre-emphasis circuit is further configured to evaluate values ofone or more adjacent bits within a predefined window.
 15. The circuit ofclaim 10, wherein said one or more predefined criteria defining saidclock-like data pattern characterize whether said data pattern exceeds apredefined frequency value.
 16. The circuit of claim 10, wherein saidone or more predefined criteria comprises one or more predefined datapatterns.
 17. The circuit of claim 10, wherein said one or morepredefined criteria characterize whether said data pattern is a clockpattern or said clock-like data pattern within an observation window.18. The circuit of claim 10, wherein a total number of pre-emphasislevels is comprised of said higher pre-emphasis level for saidclock-like data patterns and said pre-emphasis levels for said datapatterns that do not satisfy said one or more predefined criteria andwherein said total number of pre-emphasis levels exceeds a total numberof bits in an observation window.
 19. An integrated circuit, comprising:a circuit for compensating for channel distortions, comprising: a datapattern detector configured to evaluate a data pattern to betransmitted; and a pre-emphasis circuit configured to: determine if saiddata pattern satisfies one or more predefined criteria defining aclock-like data pattern; and generate a pre-emphasis level for saidclock-like data patterns that is higher than a pre-emphasis level forsaid data patterns that do not satisfy said one or more predefinedcriteria.
 20. The integrated circuit of claim 19, wherein saidpre-emphasis circuit is further configured to determine if said datapattern satisfies said one or more predefined criteria defining saidclock-like data pattern within a predefined window size.